A number of processes for the polymerization of monoolefins such as ethylene and propylene are run on a continuous basis with monomers and catalyst being fed to the reaction zone on an essentially-continuous basis with polymerization products being withdrawn from the reactor at essentially the same rate as monomer is charged to the reactor. In many of such olefin polymerization processes the catalyst employed is a finely-divided solid such as chromia supported on silica, or a catalyst prepared by reducing a transition metal compound such as TiCl.sub.3 with an aluminum alkyl. Such catalyst solids are conventionally suspended or slurried in a liquid hydrocarbon for feed to the polymerization reactor.
From theoretical considerations, it would be desirable to feed such a catalyst slurry to the polymerization reactor on a truly continuous basis. The rate of catalyst consumption in such reactions, howver, is sufficiently low that it is extremely difficult to feed such small quantities of catalysts to the reactor at steady rates. Accordingly, a standard practice has developed in which larger quantities of the catalyst are fed to the reactor at periodic intervals.
By reason of the fact that the catalyst is fed to the reactor as a slurry of finely-divided solids in an inert hydrocarbon liquid, problems are encountered in feeding substantially identical quantities of the catalyst solids to the reactor. This results from the fact that it is difficult to maintain the catalyst solids uniformly slurried throughout the hydrocarbon liquid, particularly during the time interval in which the catalyst slurry is being fed from the catalyst slurry holding tank to the polymerization reactor.
Numerous types of equipment and processes have been suggested to obtain equal quantities of catalyst solids in each lot of catalyst slurry that is periodically fed to the polymerization reactor. To date, however, none of the proposed systems operates as efficiently as desired. It is observed in actual practice that the quantity of catalyst solids fed to the reactor varies from charge to charge. For this reason, there is a need in the art for improved apparatus and methods to feed more uniform quantities of catalyst solids to olefin polymerizations.